Method for forming an insulating film on an oriented silicon steel sheet

ABSTRACT

A method for producing an oriented silicon steel sheet which a surface film, which improves iron loss and magnetostriction characteristics of the steel sheet, comprising the steps of applying to the surface of the oriented steel sheet a coating solution composed of 4 to 16 wt. percent of colloidal silica, 3 to 24 wt. percent of aluminum phosphate and 0.2 to 4.5 wt. percent of at least one compound selected from the group consisting of chromic anhydride and chromate with or without the addition of 1 to 5 grams of boric acid and baking the thus applied coating solution at a temperature above 350*C.

Tanaka et a1.

1 11- 3,856,568 [451 Dec. 24, 1974 METHOD FOR FORMING AN INSULATING FILMON AN ORIENTED SILICON STEEL SHEET Inventors: Osamu Tanaka; TakaakiYamamoto; Toshihiko Takata, all of Kitakyushu, Japan Nippon SteelCorporation, Tokyo, Japan Filed: Sept. 27, 1972 Appl. No.: 292,715

Assignee:

Foreign Application Priority Data Sept. 27,1971 Japan 46-75233 11.8. C1.117/70 B, 117/70 C, 117/129, 117/l35.1, 117/234", 148/6.16, 148/12,336/218 Int. Cl. C220 39/46, HOlf 27/24 Field of Search..... 117/129, 70C, 70 B, 1351,

References Cited I UNITED STATES PATENTS 8/1951 Thompson ..1 148/6 .l6

THE PRESENT INVENTION COATING OF MAGNESIUM 3,150,015 9/1964 Boyer et a1.l17/l35.1 X 3,207,636 9/1965 Wada et'al l48/6.l6 3,522,108 7/1970Yamamoto et al. 117/70 C X 3,522,113 7/1970 3,528,863 9/1970 3,533,86112/1970 Foster et a1 117/129 X 3,670,278 6/1972 Foster et a1 117/129 XPrimary Examiner-Ralph Husack Attorney, Agent, or FirmWenderoth, Lind &Ponack ABSTRACT A method for producing anorientedsilicon steel sheetwhich a surface-film, which improves iron loss and magnetostrictioncharacteristics of the steel sheet, comprising the steps of applying tothe surface of the oriented steel sheet a coating solution composed of 4to 16 wt. percent of colloidal silica, 3 to 24 wt. percent of aluminumphosphate and 0.2 to 4.5 wt. percent of atleast one compound selectedfrom the group consisting of chromic anhydride and chromate with orwithout the addition of 1 to 5 grams of boric acid and baking the thusapplied coating solution at a temperature above 350C.

5 Claims, 1 Drawing Figure COATING ACCORDING TO 450 IS/5o PHOSPHATE 41.3 W 6 17 A 0 /50 A o/ 1 5 1 /A 0 z A Z o 1 K O 1- 1 9' 1 1 o o 0 LI 0t: L! o I ,6

1.0- WIS/5O Ll. t 4, A a O O-9 .o J /A g A /t O: l

IRON LOSS BEFORE THE COATING tw/K PAIENTEI] DEC 2 4 I974 IRO LOSS AFTERTHE COATING (IN/K FI-I OSPHATE I7/ A y I I I I I I 0,9 LO I.I L2 L3 IRONLOSS BEFORE THE COATING Iw/x METHOD FOR FORMING AN INSULATING FILM ON ANORIENTED SILICON STEEL SHEET This invention relates to the production ofan oriented silicon steel sheet having a surface film which improves theiron loss and magnetostriction characteristics of the direction siliconsteel sheet.

It has been recognized since long ago that the magnetostriction of anoriented silicon steel sheet is one of the main causes of the noise of atransformer. made of this steel sheet. Reducing the magnetostrictionoscillation induced by the magnetostriction is important to subdue thenoise of the transformer, and the surface film formed on the steel sheetis known as an industrially effective means to this purpose. The actionof this surface film is caused by the tension exerted by the surfacefilm on the steel sheet, because in the magnetization of the orientedsilicon steel sheet under this tensionthe magnetization process, whichdoes not take part in the magnetostriction, that is, that which iscaused by the movement of the magnetic wall by 180, becomes dominant.This is explained from the view point of the magnetic section theory.The tension exerted by the surface film on the steel sheet is producedduring the steel sheet being cooled in the heat treatment due to thedifference in the thermal expansion coefficient between the steel sheetand the surface film.

" Further, the tension of the surface film on the steel sheet iseffective also to the improvement of the iron loss of the orientedsilicon steel sheet. It has been discovered by one of the presentinventors that particularly the more complete the orientation, thehigher the effect [see Jr. 'Appl. Phys, 41, 2981 (1970)]. v

The term oriented steel sheet used here means a single oriented steelsheet of Fe-Si alloy containing up to 6 percent by weight of Si ormostcommonly about 3 percentby weight of Si, having a so-called cube-onedgeor a (110) [001] crystal structure as repeated by Miller index obtainedby a combination of proper known rolling and heat-treatment of the steelsheet of this composition, in which the rolling direction ismagnetically most easy to' magnetize. There are marketed, for instance,under the trade marks of Orient core (Trade Mark of Nippon SteelCorporation) or Orient core-HI'B" (Trade Mark of Nippon SteelCorporation), which hasmore complete orientation than before (see U.S.Pat. Nos; 3,159,511 and 3,287,183).

It is usual that the surface film made on an oriented silicon steelsheet consists of a glassy film formed during the high temperaturefinishing annealing or a phosphate film applied onto the glassy film ordirectly onto the naked steel sheet having no glassy film.

The glassy film is formed by the reaction of magnesia, which is anannealing separating agent, or an oxide added, as required, to themagnesia and a surface oxidized layer of the steel sheet, and consistsmostly of magnesium silicate. The phosphate film is made by applying anaqueous solution of such metallic phosphate as magnesium phosphate oraluminum phosphate to a steel sheet and baking the same.

it has been found that among the above-mentioned surface films theglassy film has an action effective to the improvement of the iron lossand magnetostriction characteristics. However, the formation of thisfilm is so easily influenced by the characteristics of magnesia, thestate of the surface oxidized layer of the steel sheet, theannealingatmosphere and temperature conditions that it is difficult infact to obtain a uniform thickness 2 4 and characteristics. Therefore,with only the glassy film, it is insufficient to utilize the effect ofthe surface film. On the other hand, the phosphate film is low in theeffect of the surface film and may even occasionally deteriorate thecharacteristics to be lower than in case the surface film is made of theglassy film only.

There have been suggested various methods for developing the effect ofthe surface film more than ever. For example, in Japanese PatentPublication No. 32815/1970 (U.S. Pat. No. 3,522,l 13), there was formeda potassium silicate glass film on a silicon steel sheet covered with aglassy film and/or phosphate coating film. In Japanese PatentPublication No. 18605/1971 (U.S. Pat. No. 3,528,863), a glass suspensionmade by crushing a glass frit lower in the thermal expansion coefficientthan the silicon steel is applied and baked on a complex compound havingmagnesium as a substrate. Further, a method, wherein a glass frit isceramic-coated in the same manner as in the latter, is suggested inBritish Pat. No. 1,077,377.

An object of the present invention is to provide an orientedelectromagnetic steel sheet, on which is formed a surface film servingto improve the iron loss and magneto-striction characteristics.

} Another object of the present invention is to provide an orientedelectromagnetic steel 'sheet, on which is formed a surface film havinggood adhesive properties.

A further object of the present invention is to obtain a uniform surfacefilmon the above-mentioned steel sheet.

Other objects will become clear from the below described matters andaccompanying drawing.

The accompanying drawing is a graph showing the difference in the effecton the iron loss between the coating of the present invention and .aconventional magnesium phosphate coating applied to bare oriented steelsamples after having removed the surface film formed on the siliconsteel sheet products respectively.

It has been discovered by the present inventors that a surface filmformed by applying and baking'a coating solution having colloidal silicaas a main component, aluminum phosphate and at least one compoundselected from the group consisting of chromic anhydride and chromates asa binder in the fundamental composition has a great effect on theimprovement. of the iron loss and magnetostriction characteristics of anoriented silicon steel sheet. An aqueous dispersion ofsupermicro-granular (for example, of a granule diameter of 10 to 20 mp.)colloidal silica performs a film formation. When the aqueous solution isapplied onto a steel sheet and isbaked, a film can be formed. However,this film has a disadvantage that it is low in the adhesion to the steelsheet. If a mixed aqueous solution of aluminum phosphate and one or moreof'chromic anhydride and chromates is added to this aqueous dispersionof colloidal silica, the adhesion of the film can be improved. This isthought to be attributable to the fact that the above-mentioned mixtureacts as a binder. Even with aluminum phosphate alone as a binder, theadhesion may be improved but, when the film is long exposed to a wetatmosphere, there occurs a phenomenon that the silica lamina isisolated. In order to prevent this phenomenon, it is effective to addone or more of chromic anhydride and chromates to the solution.

The reason, why the effects of improving iron loss and magnetostrictioncharacteristics of an oriented silicon steel sheet, obtained by asurface film composed of colloidal silica as a main component of thecoating solution of the present invention become very high, as shown inthe later described examples, resides in the following facts, that is,due to the low thermal expan sion coefficient of silica in general thesurface film composed of colloidal silica as a main component is alsosolow in the thermal expansion coefficient that the tension exerted bythe surface film on the steel sheet is high, and further due to acharacteristic action of supermicrogranular colloidal silica of fillingminor defects of the surface any unevenness of the glass film can becorrected.

Further, this surface film has not only the abovementioned features butalso the following features for the oriented silicon steel sheet. Evenwhen thickly applied to, it presents aa beautiful appearance and theadhesion is not impaired thereby. Therefore, a high interlayerresistance and voltage resistance can be easily obtained. The film isalso so flat and smooth that, even if it is thickly applied, apractically sufficient space factor can be obtained. Further, the filmis so compact that it is high in the heat-proofness,atmosphere-proofness and anticorrosion. These additional features makethe effects of improving the iron loss and magnetostrictioncharacteristics of the surface film of the present invention morepositive.

In the following a surface film of the present invention and a method ofapplying it to the production of oriented silicon steels shall beexplained in detail.

The properties and mixing rates of component solutions of the coatingsolution to be used in the present invention are as follows.Supermicrogranular colloidal silica which is dispersed in water and is amain component of the coating solution is required to obtain a sta blemutual solubility with an aqueous solution of a phosphate, chromic acidor a chromate as a binder. For instance, commercial Snowtex (Trade Markof Nissan Chemical Industries, Ltd.) (of an SiO content of 20 percent,hydrogen ion concentration (pH) of 3.0 to 4.0 and specific gravity of1.15 at 20C.) renders a suitable example for the present invention.Aluminum phosphate to be used as a binder may be nearly of a compositionof aluminum biphosphate, in which a mixing ratio of A1 0 and H PO isabout 0.16 by mols and the proper pH is about 0.9 (about 1.9 ascorrected by the concentration) in an aqueous solution of 50 percent. Itis needless to say that the present invention is not limited to theabove-mentioned values. However, according to experiences, when the pHis low, the solution bubbles and its applicability is reduced. In orderto obtain the above-mentioned pH value, the said mol ratio is suitablefor the preparation and there is further an advantage that a commercialproduct can be utilized.

The proper mixing ratios of components of the coating solution of thepresent invention are as follows: 20 to 80 parts by volume of a 50percent aqueous solution of aluminum phosphate are added to 100 parts byvolume of a 20 percent aqueous dispersion of colloidal silica. Thismixture corresponds to a coating solution composed of to 16 percent byweight of colloidal silica and 9 to 24 percent by weight of aluminumphosphate (as calculated as aluminum biphosphate here and hereinafter).'Ifthe aqueous solution of aluminum phosphate is less than parts byvolume, there is shown a phenomenon that, after the baking, the adhesionof the above described silica lamina becomes insufficient and,

if it is more than parts by volume, after the baking, the film becomesturbid to be white and shows a bad appearance. Chromic acid is ofchromic anhydride powder. The addition of 1 gram of chromic anhydridepowder to cc. of an aqueous dispersion of colloidal silica can presentthe above described isolation of the silica lamina which is to be causedin the case of adding no chromic anhydride powder. The proper range ofthe addition of chromic anhydride powder is 3 to 9 grams. In the case ofadding chromate, for instance, more than 5 parts by volume of a 25percent aqueous solution of magnesium chromate are to be added to 100parts by volume of colloidal silica. Then, the above-mentioned isolationof the silica lamina may be prevented. The preferable range of theaddition of chromate is 10 to 30 parts by volume.

The coating solution of the above-mentioned compositioncorresponds to anaqueous solution of about 8 to 16 percent by weight of colloidalsilica,7 to 24 percent by weight of aluminum phosphate and 0.4 to 4.5 percentby weight of chromic anhydride or magnesium chromate. Further, asrequired, in case a thus coated steel sheet is likely to stick duringthe heat-treatment, for instance, in case the heat-treatment is carriedout at a temperature above 800C, it will be effective to add boric acidor supermicro-granular silica. It is proper to add 1 to 5 grams of boricacid or 0.25 to 2 grams of such supermicrogranular silica as, forexample, commercial Nipsil VN 3 (Trade Mark of Nippon Silica IndustrialCo., Ltd.) (of 93 to 94% SiO a granule diameter less than 10 p. and a pH(at 4 percent) of 5.8 to 6.8) to 100 cc. of an aqueous dispersion ofcolloidal silica.

The addition of boric acid or supermicrogranular silica is carried outas follows:

A. Case of adding boric acid For example, in the mixing rates of 100 cc.of an I aqueous dispersion of colloidal silica,'60 cc. of an aqueoussolution of aluminum phosphate, 6 grams of chromic anhydride and 2 gramsof boric acid:

i. 2 kg. of boric acid are put into 15 liters of warm water above 60C.and are well stirred to be dissolved. (This shall be a solution A.)

ii. 6 kg. of chromic anhydride are added to 60 liters of an aqueoussolution of 50 percent aluminum phosphate and are well stirred to bedissolved and then 100 liters of an aqueous dispersion of colloidalsilica are added to the solution. (This shall be a solution B.)

iii. The solutions A and B are mixed and stirred.

The thus prepared solution is of a concentration of about 23 to 28 B.

B. Case of adding supermicrogranular silica For example, in the case ofthe mixing rates of 100 cc. of an aqueous dispersion of colloidalsilica, 60 cc.

of an aqueous solution of aluminum phosphate, 6

grams of chromic anhydride and 0.5 gram of supermicrogranular silica:

6 kg. of chromic anhydride are added to 60 liters of a 50% aqueoussolution of aluminum phosphate and are well stirred to be dissolved andthen 100 liters of an aqueous dispersion of colloidal silica and 0.5 kg.of supermicrogranular silica are added and stirred.

The thus prepared solution is of a concentration of about 26 to 31 B.

In applying the coating solution of the abovementioned composition to anoriented silicon steel sheet, it is used after adjusted to be of aconcentration proper for the objective film deposition amount. That isto say, it is used in the state of the original solution so as tocontain 4 to 16 percent by weight of colloidal silica, 3 to 24 percentby weight of aluminum phosphate and 0.2 to 4.5 percent by weight of oneor more of chromic anhydride and chromates or as diluted with water.

The coating solution of the present invention is uniformly applied ontothe surface of the abovementioned oriented silicon steel sheet bydipping or spraying and thereafter is pressed with pressing rolls, orcoated by any known method.

The coating solution of the present invention can be applied to anoriented silicon steel sheet, irrespective of the surface ofthe orientedsilicon steel sheet being covered with a glassy film or with a phosphatefilm or both films or being bare without covered by them.

However, when applying the coating solution of the present invention nophosphate is required. Phosphate film is not only uneconomic, but ratherdeteriorates the properties inherent to the coating'of the presentinvention, excepting a special case of thinly applying the coatingsolution of the present invention for the purpose of supplying adeficiency of the phosphate film. Such an oriented silicon steel sheethaving a bare surface is often produced in order to improve thepunchability.'lt is known, however, that in such a case particularlymagnetostriction characteristics of the steel sheet are remarkablydeteriorated on account of the surface having no coating. When thecoating of the present invention is applied to such steel sheet, themagnetostriction characteristics can be improved substantially to thoseof the case of being covered with a glassy film alone, as'will be seenfrom the later described examples.

The coating solution of the present invention is baked after it has beenapplied to the surface of a steel sheet as above-mentioned. A favorablesurface film can be obtained, when it is heated to a temperature above350+C., preferably a temperature between 400to 900C. It is a greatadvantage of the coating solution of the present invention that thebaking temperature range is so wide as above-mentioned. On account ofthis the baking treatment can be carried out by various methodsaccording to the object to be obtained. The lower limit of the bakingtemperature is set at 350C, because, if the coating temperature is belowthis limit, the reaction of the coating solution is insufficient and thehygroscopicity remains in the formed surface film. As to the upper limitit is to note that the baking temperature may be raised up to arelatively high one, but practically the temperature above 900C. is noteconomical, and it is even feared that at such a high temperature above900C. the characteristics of the surface film will be deteriorateddepending upon the atmosphere and treating time. Thus, the bakingtemperature above 900C. is not desirable.

The atmosphere at the time of baking may be of air, nitrogen or amixture of nitrogen and hydrogen.

It has been found that, when the baking is carried out on the hightemperature side of the above-mentioned temperature range or even incase it is carried out at a low temperature, if a strain-removingannealing is further carried out continuously or by batch at about 800C.as carried out by an ordinary consumer, the effects of improving theiron loss and magnetostriction characteristics of the surface film ofthe present invention can be increased. From the fact that the tensionof the surface film on the steel sheet is caused by the difference inthe thermal expansion coefficient, it is naturally presumed to beeffective to bake it at a high temperature. Further, in the case of thebaking being carried out at low temperature, it is thought that asurface film effective to the tension may be re-formed in the later hightemperature treatment.

ln the production of an oriented silicon steel sheet, it is economicalto continuously carry out the step of forming a surface film from thecoating solution of the present invention in combination with othersteps, Generally, the glassy film on an oriented silicon steel sheet isused to be formed by a high temperature finishing annealing. Then, theexcess annealing separating agent is removed by water-washing or lightpickling to leave the glassy film on the surface of the steel sheet. Or,for the purpose of improving the punchability, the glassy film may bealso'removed by pickling. Then, a so-called heat-flattening is made toremove a coil bent of the steel sheet and to flatten the steel sheet.This heat-treatment is carried out at 800 to 900C. for not more thanabout 3 minutes. This condition corresponds to the high temperature sideof the above-mentioned baking condition. Therefore, if the coatingsolution is applied before the heat-flattening, the heat-flattening andbaking are able to be simultaneously carried out. Further, as in suchcase, the interlayer resistance and voltage resistance are oftenimpaired by flaws caused by builds-up on hearth rolls, the coatingsolution may be again applied and baked at, a low temperature in orderto remedy the above-mentioned defect. Or, it is also possible to applythe coating solution and baking the same at a low temperature of 400 to500C. after the heat-flattening. In such case, a sufficient interlayerresistance and voltage resistance can be easily obtained, but, in orderto more sufficiently develop the effects of improving the iron loss andmagnetostriction characteristics by the surface film, it is necessary toanneal the f lm at about 800C. continuously or by batch by a consumer ormanufacturer.

The above-mentioned respective steps can be carried out continuously onone continuous line but, needless to say, may be carried out on separatelines as divided into any number of steps.

Examples of the present invention are shown in the following:

EXAMPLE 1 Samples taken adjacently to one another from the samecommercial coil of an oriented silicon steel sheet of a thickness of0.30 mm. were treated with a mixed acid of sulfuric acid andhydrofluoric acid to remove the surface films an'd were thencontinuously annealed at 800C. in hydrogen for 3 minutes to removestains. Thus,'the samples for the test have been prepared. To thesesamples there were applied a phosphate coating solution of magnesiumphosphate and a coating solution of a composition of cc of a 20 percentaqueous dispersion of colloidal silica, 60 cc. of an 50 percent aqueoussolution of aluminum phosphate, 6 grams of chromic anhydride and 2 gramsof boric acid of the present invention respectively, and were then bakedat 800C. for 15 seconds in a nitrogen atmosphere in a continuous oven sothat the coating deposition might be 4 gr./m on one surface. Themagnetic properties and magnetostriction characteristics were measuredwith a single plate measuring device before applying the coating andafter having applied and baking it. The sample was of a length (in therolling direction) of 50 cm. and a width of cm. 1

The Figure shows the results of the measurements of iron losses /50 andW 17/50 before and after forming films of respective coatings. Thereduction in iron loss obtained by the coating solution of the presentinvention is very large. 1 Table 1 indicates the results of themeasurements of the magnetostriction characteristics. The sample by thecoating solution of the present invention is low in the magnetostrictionand is also low in the deterioration to be caused by the compressiveforce.

The above-mentioned results demonstrate that the effects of improvingthe iron loss and magnetostriction characteristics of the surface filmby the coating solution of the present invention are large.

Table 1 Samples Magnetic Magnetostriction Magnetostriction permeability(X 10 (X 10 at 17 kg. at 10 Oe under compression 17 kg. 19 kg. of 30kgJcm Before applying the coating:

After applying and baking the coating of the present invention:

Before applying the coating:

After applying and baking magnesium phosphate coating;

Example 2 Samples taken adjacently to one another from the same coil ofan oriented silicon steel sheet of a thickness of 0.30 mm., as finishedannealed at a high temperature, were subjected to water-washing andlight pickling to remove an annealing separating agent remained on thesurfaces of the samples and to leave the glassy films and were thenannealed to remove coil bents and strains. Thus, the samples for thetest were prepared. To these samples covered with the glassy films therewere applied a phosphate coating solution of magnesium phosphate and acoating solution of 100 cc. of a percent aqueous dispersion of colloidalsilica,'60 cc. of a 50 percent aqueous solution of aluminum phosphate, 6grams of chrornic anhydride and 0.75 gram of supermicrogranular silicaof the present invention respectively in the same manner as in Example 1and were thereafter baked at 850C. for 10 seconds in air in a continuousoven.

Table 2 shows the results measured in the same manner as in Example 1before applying the coating and after applying and baking it. The effectof the surface filmby the coating solution of the present invention islarge.

In Table 3 there are shown characteristics of the surface film samplesby the coating solution of the present invention after subjected to astrain-removing annealing at 800C. for 4 hours in a dry atmosphere of10% H and N The effect of the surface film is not lost even by theheat-treatment of a long time.

Table 2 Sam- Magnetic .lron loss (/kg.) Mugnetostriction plespermeability (X 10") at 10 Oe W 15/50 17/50 17 kg. 19 kg.

1n the case of glassy film only:

1 1913 0.850 1.155 0.7.6 +1.82 2 1921 0.843 1.150 0.13 +1.69 3 19160.833 1.131 0.39 +1.43 After applying and baking the coating of thepresent invention:

1 1916 0.803 1.082 0.52 +0.78 2 1924 0.793 1.075 0.39 +0.52 3 1920 0.8031.086 -0.59 +0.65 1n the case of glassy film only:

4 1930 0.870 1.203 0.13 +1.95 5 1927 0.850 1.139 -0.26 +1.56 6 19270.828 1.106 0.39 +1.30 After applying and baking magnesium phosphatecoating:

Table 3 After annealing in 10% H and 90% N at 800C. for 4 hours Sam-Magnetic lron loss ('lkg.) Magnetostriction ples permeability (X 10") at10 0e -"'15/50 "'17/50 17 kg. 19 kg.

After applying and baking the coating of the'present invention:

1 1919 0.803 11085 -0.52 +0.71 2 1925 0.795 11075 0.39 +0.52 3 19230.802 11084 0.59 +0.59 After applying and baking magnesium phosphatecoating:

Table 4 shows the results of measuring the coating deposition,interlayer resistance and-space factor of' these surface films. Thesurface film of the present invention is high in the interlayerresistance and is very small particularly in the reduction in resistanceto be caused by the annealing in a reducing atmosphere. Though it ishigh in the coating deposition, it keeps a high space factor.

These results show that the surface film of the present invention isvery smooth and compact.

Table 4 Distribution of interlayer resistance Samples Deposition SpaceJust after coating:

After annealing in 10% H and 90% N at; 800C. for 4 hours (Q-cm ./sheet)4.9 .O-9 .9 .0-19 .9 20 .O-49 .9 5O .O-99 .9 100- Coating of the presentinvention:

Magnesium phosphate coating:

EXAMPLE 3 1 Samples were prepared-in the same manner as in Example 2. Acoating solution of a composition of 100 cc. of a 20 percent aqueoussolution of colloidal silica, 60 cc. ofa 50 percent aqueous solution ofaluminum phosphate, cc. of a 25 percent aqueous solution of magnesiumchromate and 0.75 gram of supermicrogranular silica of the presentinvention was applied to these samples having glassy films andwas bakedat about 450C. for 10 seconds in an open oven. These samples werefurther continuously annealed to remove strains at 810C. for 2 minutesin a nitrogen atmosphere.

Table 5 shows the results measured in the same manher as in Example 1before and after the strainremoving annealing. The improvements of theiron loss and magnetostriction characteristics after the annealing arelarge, showing the better characteristics than in the case of the glassyfilm only mentioned also in the same table. This shows that, even whenthe coating solution of the present invention is baked at a lowtemperature, a sufficient effect of the surface film can be obtained bya subsequent annealing at a high temperature.

Table 5 Sam- Magnetic lron loss /kg.) Magnetostriction ples permeability(X 10') at 10 0e 15/50 17/50 17 kg. 19 kg.

1n the case of glassy film only:

1 1946 0.809 1.099 -0.13 +1.30 2 1911 0.843 1.158 0.26 +2.08 I 3 19260.800 1.099 0.26 +1.82 Afterapplying the coating of the presentinvention and baking it at a low temperature:

1 1945 0.812 1.102 +0.13 +1.56 2 1911 0.845 1160 +0.26 +2.21 3 19250.803 1.101 +0.19 +2.08 After continuous annealing in N at 810C. for 2minutes: 1 1947 0.780 1.046 0.52 +0.52 2 1913 0.815 1.108- 0.65 +0.78 31929 0.778 1.056 0.52 +0.59

EXAMPLE 4 After the treatment in Example 2, a coating solution of 100cc.'of a 20 percent aqueous dispersion of colloidal silica, 60 cc. of a50 percent aqueous solution of aluminum phosphate and 15 cc. of apercent aque 1 B7 .0 l .9 1 1 .1 50.0 I 33 .3 3 .7

ous solution of magnesium chromate of the present invention (theoriginal solution was of about 25 Be) as diluted with water to be ofabout 15 Be was applied to the steel sheet and was baked at 400C. for 10seconds in an open oven. The obtained surface film was flat, smooth anduniform.

This shows that the coating solution of the present invention can beused also for improving the interlayer resistance and voltage resistanceand further for recoating for the purpose of securing the insulation ofthe part, after a protrusion thereof has been removed by shearing orslitting.

What is'claimed is:

1. A method for forming an insulating film on an oriented silicon steelsheet, comprising the steps of applying to the oriented silicon steelsheet a' coating solution composed of 4 to 16 wt. percent of colloidalsilica, 3 to 24 wt. percent of aluminum phosphate, calculated asaluminum biphosphate, and 0.2 to 4.5 wt. percent of at least onecompound selected from the group consisting of chromic anhydride andchromate and baking the thus applied coating solution at a temperatureabove 2. The method according to claim 1, in which boric acid is addedto the coating solution in an amount of 1 to 5 grams per cc. of a waterdispersion of colloidal magnetostriction characteristic of the steelsheet, comprising the steps of forming a glassy film. on the orientedsilicon steelsheet by a reaction with an annealing separating agentduring a high temperature finishing annealing of said steel sheet,thereupon applying to this glassy film a coating solution composed of 4to 16 wt. percent of colloidal silica; 3 to 24 wt. percent of aluminumphosphate, calculated as aluminum biphosphate, and 0.2 to 4.5 wt.percent of at least one compound selected from the group consisting ofchromic anhydride and chromate. baking the thus applied coating solutionat a temperature above 350C and then subjecting the magneto-strictioncharacteristics of the steel sheet, ac

cording to claim 4, in which the glassy film formed on the surface ofthe steel sheet during the high temperature finishing annealing isremoved by a pickling and then the coating solution is applied to thesurface of the steel sheet.

1. A METHOD FOR FORMING AN INSULATING FILM ON AN ORIENTED SILICON STEEL SHEET COMPRISING THE STEPS OF APPLYING TO THE ORIENTED SILCON STEEL SHEET A COATING SOLUTION COMPOSED OF 4 TO 16 WT. PERCENT OF COLLOIDAL SILICA, 3 TO 24 WT. PERCENT OF ALUMINUM PHOSPHATE, CALCULATED AS ALUMINUM BIPHOSPHATE AND 0.2 TO 4.5 WT. PERCENT OF AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF CHROMIC ANHYDRIDE AND CHROMATE
 2. The method according to claim 1, in which boric acid is added to the coating solution in an amount of 1 to 5 grams per 100 cc. of a water dispersion of colloidal silica.
 3. The method according to claim 1, in which supermicrogranular silica is added to the coating solution in an amount of 0.25 to 2 grams per 100 cc. of a water dispersion of colloidal silica.
 4. A method for producing an oriented silicon steel sheet with a surface film which improves iron loss and magnetostriction characteristic of the steel sheet, comprising the steps of forming a glassy film on the oriented silicon steel sheet by a reaction with an annealing separating agent during a high temperature finishing annealing of said steel sheet, thereupon applying to this glassy film a coating solution composed of 4 to 16 wt. percent of colloidal silica, 3 to 24 wt. percent of aluminum phosphate, calculated as aluminum biphosphate, and 0.2 to 4.5 wt. percent of at least one compound selected from the group consisting of chromic anhydride and chromate, baking the thus applied coating solution at a temperature above 350*C and then subjecting the steel sheet to a heat-treatment in a temperature range of from 800* to 900*C after or during the said application of the coating solution and baking thereof.
 5. The method for producing an oriented silicon steel sheet with a surface film which improves iron loss and magneto-striction characteristics of the steel sheet, according to claim 4, in which the glassy film formed on the surface of the steel sheet during the high temperature finishing annealing is removed by a pickling and then the coating solution is applied to the surface of the steel sheet. 